(1) Field of the Invention
The present invention relates to a mobile terminal apparatus and a channel compensation method of the mobile terminal apparatus. The present invention relates to, for example, a preferable art employed in an apparatus for communicating by an HSDPA (High Speed Downlink Packet Access) transmission system, which is one of radio mobile terminal transmission systems.
(2) Description of Related Art
An HSDPA for providing maximum transmission rate of 14 Mbps in downlink communications from a base station apparatus (hereinafter, also referred to as “base station”) to a mobile terminal apparatus (hereinafter, referred to as “mobile terminal”) is specified in the 3GPP (the 3rd Generation Partnership Project) as a theme of standardization of W-CDMA (Wideband-Code Division Multiple Access) system, which is one of the third generation mobile communication systems.
The HSDPA transmission system is an art for changing the number of multicodes, modulating system (such as QPSK or 16QAM), transmission block size (TBS: Transport Block Size) or the like of an HS-PDSCH (High Speed-Physical Downlink Shared Channel), which will be described later, in order to select the most appropriate transmission rate and perform communications in accordance with a reception environment where a mobile terminal is located.
The HSDPA employs an adaptive coding modulation system and, for example, it is characterized by adaptively switching QPSK modulation system and 16QAM system according to radio environment between a base station and a mobile terminal. Further, in order to realize the adaptive coding modulation system, a CQI (Channel Quality Indicator) for reporting reception environment from the mobile terminal to the base station is defined and formats of different transmission speed are defined as a CQI table according to the cases of CQI=1 to 30, for example.
The mobile terminal measures reception environment and, when assuming that an HS-PDSCH is received within 3 slots since a slot before CQI transmission under such environment, the mobile terminal reports a CQI which is the largest but below ‘HS-PDSCH BLER (Block Error Rate)=0.1’ or a CQI lower than that to the base station.
As major radio channels employed in the HSDPA, there are HS-SCCH (High Speed-Shared Control Channel), HS-PDSCH (High Speed-Physical Downlink Shared Channel), and HS-DPCCH (High Speed-Dedicated Physical Control Channel).
The HS-SCCH and the HS-PDSCH are both downlink shared channels and the HS-SCCH is a control channel for transmitting various parameters related to data transmitted on the HS-PDSCH. The various parameters include, for example, modulating type information indicating which modulation system is employed to transmit data on the HS-PDSCH, the number of allocation of spread coding (the number of codes), or pattern of rate matching process performed on transmission data.
The HS-DPCCH is an uplink dedicated control channel from the mobile terminal to the base station. The HS-DPCCH is used when the mobile terminal transmits an ACK signal and a NACK signal to the base station according to a result of data reception of HS-PDSCH.
The HS-DPCCH is also used when the mobile terminal measures reception quality (for example, SIR: Signal Interference Ratio) of the reception signal from the base station and periodically transmits the result to the base station as CQI (see FIG. 9). The base station determines the downlink radio environment based on the received CQI. When the environment is good, the base station may switch to a modulation system for faster data transmission rate and when the environment is not good, the base station adaptively switches to a modulation system for slower data transmission rate.
(Channel structure)
A channel structure of HSDPA will be described.
FIG. 9 is a diagram showing a channel structure of HSDPA. It is noted that, in W-CDMA system, each channel is separated by coding to be adapted to code division multiplex system.
Firstly, channels, which are yet to be described among the channels shown in FIG. 9, will be described.
A CPICH (Common Pilot Channel) and a P-CCPCH (Primary Common Control Physical Channel) are respectively downlink shared channels. The CPICH is a channel used for channel estimation, cell search, and a timing basis of other downlink physical channels in the same cell and used to transmit so-called pilot signals (known signals between the base station and the mobile terminal). The P-CCPCH is a channel for transmitting broadcasting information.
Next, timing relationship in each channel will be described.
As shown in FIG. 9, each channel includes a frame (10 ms) that is composed of 15 slots. As described above, the CPICH is used as a basis of other channels and the beginning of frames of the P-CCPCH and HS-SCCH is respectively corresponding to the beginning of a frame of the CPICH. Here, the beginning of a frame of the HS-PDSCH is delayed by 2 slots with respect to that of HS-SCCH.
This delay is provided in order to notify, in advance, modulating type information or spread code information, via the HS-SCCH, which are required for demodulating the HS-PDSCH in the mobile terminal.
Accordingly, the mobile terminal performs HS-PDSCH demodulation or the like by selecting the corresponding demodulating system and despreading code according to the notified information via the HS-SCCH. Further, the HS-SCCH and the HS-PDSCH includes a sub-frame composed of 3 slots. The foregoing is the brief description of the HSDPA channel structure.
(Structure of Mobile Terminal)
FIG. 10 shows an example of a structure of a relevant part of a known mobile terminal (mobile terminal apparatus) adapted to HSDPA. As shown in FIG. 10, the mobile terminal includes, for example, a receiver 101, a CQI reporting value calculator 102, an HS-SCCH channel estimation filter 103, an HS-SCCH channel compensator 104, an HS-SCCH demodulator 105, an HS-SCCH decoder/CRC calculator 106, an HS-PDSCH symbol buffer 107, an HS-PDSCH channel estimation filter 108, an HS-PDSCH channel compensator 109, an HS-PDSCH demodulator 110, an HS-PDSCH decoder 111, an HS-PDSCH-CRC calculator 112, a downlink reception timing monitor 113, an uplink transmission timing manager 114, a scheduler 115, an encoder 116, a modulator 117, and a transmitter 118.
In the mobile terminal shown in FIG. 10, a reception signal received by a reception antenna (not shown) is input into the receiver 101. The receiver 101 performs processes such as path detection or despreading for downlink and separates each channel of CPICH, HS-SCCH, and HS-PDSCH. The separated CPICH is input into the CQI reporting value calculator 102, the HS-SCCH channel estimation filter 103, and the HS-PDSCH cannel estimation filter 108, respectively.
The CQI reporting value calculator 102 obtains a reception SIR based on a pilot signal (CPICH symbol) received via CPICH and calculates a CQI reporting value corresponding to the reception SIR. The HS-SCCH channel estimation filter 103 and the HS-PDSCH channel estimation filter 108 calculate channel estimation values of HS-SCCH and HS-PDSCH respectively based on to the reception pilot signals.
On the HS-SCCH separated in the receiver 101, the HS-SCCH channel compensator 104 performs channel compensation in use of a channel estimation value obtained in the HS-SCCH channel estimation filter 103, the HS-SCCH demodulator 105 performs demodulation, and the HS-SCCH decoder/CRC calculator 106 performs decoding and CRC calculation (error check). Since the information decoded in the HS-SCCH decoder/CRC calculator 106 includes, as described above, information required for HS-PDSCH decoding such as modulating type information and spread code information, it is provided to the HS-PDSCH decoder 111. Here, when the result of HS-SCCH CRC calculation is NG, an error (DTX) is notified to the scheduler 115 in order to notify the base station.
On the other hand, the reception signal of the HS-PDSCH (HS-PDSCH symbol) which is separated in the receiver 101 is firstly buffered and delayed in the HS-PDSCH symbol buffer 107. Then, the HS-PDSCH channel compensator 109 performs channel compensation in use of the channel estimation value obtained in the HS-PDSCH channel estimation filter 108 and the HS-PDSCH demodulator 110 performs demodulation. Here, as described later with reference to FIG. 11, the HS-PDSCH symbol is delayed in the HS-PDSCH symbol buffer 107 since it is preferable to use a channel estimation value that is calculated by averaging CPICH symbols of a plurality of past and future slots with respect to a target HS-PDSCH slot of the demodulation.
On the demodulated HS-PDSCH, the HS-PDSCH decoder 111 decodes in use of spread code information obtained in the HS-PDSCH decoder/CRC calculator 106 and the HS-PDSCH-CRC calculator 112 performs CRC calculation. Then, the calculation result (OK or NG) is transmitted to the scheduler 115 as ACK/NACK.
The scheduler 115 schedules a CQI reporting value from the CQI reporting value calculator 102, DTX from the HS-SCCH decoder/CRC calculator 106, and ACK/NACK from the HS-PDSCH-CRC calculator 112, respectively, in accordance with a transmission timing signal from the uplink transmission timing manager 114. That is, as shown in the last line in FIG. 9, the scheduler 115 schedules so that the CQI reporting value is transmitted about 2.5 slots later from the reception of HS-PDSCH and ACK/NACK (/DTX) are respectively transmitted about 7.5 slots later from the completion of receiving the HS-PDSCH. Here, the reception of HS-PDSCH is monitored by the downlink reception timing monitor 113.
On each information scheduled as described above, the encoder 116 encodes as HS-DPCCH data and the modulator 117 modulates. Then, the transmitter 118 transmits that information to the base station via HS-DPCCH. The base station transmits new data when receiving ACK, retransmits HS-SCCH and HS-PDSCH when receiving DTX, and retransmits HS-PDSCH when receiving NACK.
As described above, a mobile terminal of HSDPA firstly decodes HS-SCCH and then decodes HS-PDSCH in use of the decoding result of HS-SCCH. Accordingly, in general, a channel having higher error tolerance (better reception quality) is allocated for HS-SCCH than the case of HS-PDSCH.
It is noted that there are some arts disclosed in the following patent publications (1) to (3), which are related to HSDPA.
(1) Japanese Patent No. 3471785 discloses an art in which a base station performs a reliability determination process on ACK/NACK region of uplink HS-DPCCH in order to reduce phenomena of incorrect reception of ACK which is NACK in actual. In other words, according to the art of Japanese Patent No. 3471785, cases which are likely to include an error in an uplink ACK signal are detected to revise the signal as an NACK signal, so that lack in downlink data can be improved.(2) Published Japanese translation of a PCT application, No. 2005-522911 discloses an art in which power control is performed on an ACK/NACK region of uplink HS-DPCCH (transmission power is increased when there is a possibility of incorrect reception of ACK/NACK signals) in order to reduce incorrect reception of ACK/NACK signals in a base station.(3) Published Japanese translation of a PCT application, No. 2005-510173 discloses an art to appropriately perform power control of uplink HS-DPCCH.
FIG. 11 is a diagram showing a time chart image of HS-PDSCH channel estimation and compensation performed in the mobile terminal shown in FIG. 10 by symbol unit. FIG. 12 is a diagram showing a time chart image of HS-SCCH channel estimation and compensation performed in the mobile terminal shown in FIG. 10 by symbol unit.
In FIGS. 11 and 12, the number of symbols in a single slot is defined as 10 from #0 to #9.
In HS-PDSCH modulation in the mobile terminal, in order to modulate a slot (e.g. slot #n in FIG. 11), a channel estimation value which is appropriate to the time of slot #n is required to be calculated from a CPICH symbol to carry out a modulation process on an HS-PDSCH symbol. Accordingly, the channel estimation value which is appropriate to the time of slot #n (filtering process) is preferably calculated by averaging (each “Σ” in FIGS. 11 and 12 represents an averaging process) past and future CPICH symbols (slot #n−1 to slot #n+1), however, in this case, it gets to the time of slot #n+1 before the channel estimation process is completed.
Therefore, in the mobile terminal, as indicated by an arrow 200 in FIG. 11, the HS-PDSCH symbol buffer 107 delays an HS-PDSCH symbol of slot #n and a modulation process is performed at the time from slot #n+1 to slot #n+2.
Here, it is specified that, in HSDPA, as described above, ACK/NACK signal is transmitted to the base station at 7.5 slots later from the completion of HS-PDSCH reception and HS-PDSCH is received at 2 slots later from the reception of HS-SCCH. In order to complete an HS-PDSCH decode process on a data signal transmitted by about 14 Mbps, which is the maximum throughput in HSDPA, before ACK/NACK(/DTX) transmission, information required for HS-PDSCH decoding (HS-PDSCH decode information) needs to be obtained by performing the HS-SCCH demodulate and decode processes within one slot.
Therefore, in order to demodulate a slot of HS-SCCH (e.g. slot #n in FIG. 12), for example, it is preferable to perform a modulation process in use of a channel estimation value calculated from past and future CPICH symbols with respect to the reception symbol (e.g. CPICH symbols from slot #n−1 to slot #n+1) by delaying HS-SCCH reception symbol by one slot (buffering process) similar to the demodulation process of HS-PDSCH. However, because of the above temporal restriction, the HS-SCCH reception symbol cannot be delayed (buffered).
Therefore, for demodulation of HS-SCCH on slot #n, future CPICH symbol cannot be used and a demodulation process is performed in use of a channel estimation calculated from only past CPICH symbols (e.g. CPICH symbols from slot #n−2 to slot #n).
In other words, in FIG. 11, focusing attention on symbol #0 of slot #n, channel estimation value filtering for symbol #0 of HS-PDSCH is performed in use of CPICH symbols from the first CPICH symbol in the past slot #n−1 to the last CPICH symbol in the future slot #n. Accordingly, a channel estimation value can be calculated from past and future CPICH symbols (slot #n−1 to slot #n+1) with respect to the time of symbol #0 of HS-PDSCH.
On the contrary, as shown in FIG. 12, since channel estimation filtering for symbol #0 in slot #n of HS-SCCH needs to be performed by the last CPICH symbol in past slot #n−1, the filtering is performed in use of CPICH symbols from the first symbol in slot #n−2 to the last symbol in slot #n−1.
Therefore, since a channel estimation value with respect to the time of the first symbol in slot #n−1 of HS-SCCH (one slot prior to the time of symbol #0) is calculated, a channel estimation value which is not appropriate to the time of symbol #0 may be calculated in some reception environment. As a result, the reception quality of HS-SCCH is often lower than the reception quality of HS-PDSCH in an environment in which a channel estimation result may be changed within a short time by high speed fading or the like and a past estimation value and a current channel estimation value are different.
As described above, in general, a channel having higher error tolerance is allocated to HS-SCCH than HS-PDSCH so that the reception quality is usually better in HS-SCCH than in HS-PDSCH. However, because of the temporal restriction for the demodulation process, only past CPICH symbols may be used for the channel estimation value for HS-SCCH modulation. As a result, the relation of reception quality in HS-SCCH and HS-PDSCH is reversed in some radio environment of high speed fading.
Such phenomenon will be explained with reference to FIGS. 13 and 14. FIG. 13 is a graph quantitatively showing HS-PDSCH BLER (Block Error Rate) characteristics when receiving a fixed format corresponding to a fading speed. FIG. 14 is a graph quantitatively showing HS-SCCH BLER characteristics corresponding to a fading speed.
As shown in FIG. 13, BLER of HS-PDSCH is approximately constant with respect to a fading speed; however, as shown in FIG. 14, BLER of HS-SCCH is deteriorated as the fading speed increases. In this manner, the reception quality of HS-SCCH tends to be deteriorated comparing to the reception quality of HS-PDSCH under an environment, such as fading environment, in which temporal phase changes are quantitatively generated.
Therefore, in case of high speed fading, even when the reception quality of HS-PDSCH is comparatively good, the reception quality of HS-SCCH is deteriorated. Accordingly, the CRC calculation result of HS-SCCH is determined as NG and a decode process of HS-PDSCH cannot be performed. As a result, a reception speed is reduced and throughput in the mobile terminal and throughput in the system may be decreased.
According to the arts in Japanese Patent No. 3471785, Published Japanese translation of a PCT application, No. 2005-522911, and Published Japanese translation of a PCT application, No. 2005-510173, an ACK signal which is likely to be incorrect is revised to NACK signal, or transmission power control is performed in order to reduce incorrect reception of ACK/NACK signals and lack of downlink data. However, any of those arts cannot adaptively improve the HS-SCCH reception quality itself without particular transmission power control with the base station.